In a coherent light source, such as a solid-state laser, semiconductor laser or amplifier, it is advantageous to restrict the range of operational wavelengths that propagate within the radiation source. In particular, it is important in many applications to provide a laser device that will operate in single longitudinal mode, particularly in telecommunication applications where mode hopping in the laser operation cannot be tolerated. As a result, distributed feedback (DFB) semiconductor lasers and distributed Bragg reflector (DBR) semiconductor lasers have been continually redefined and developed to provide signal and pump sources for optical data communication systems that are wavelength reliable.
Longitudinal mode control alone is not sufficient to guarantee stable single wavelength output from DFB and DBR laser devices. The existence of multiple lateral modes in these devices, particularly those with a broad area gain region, can lead to a broadened spectral output as well as instabilities in the output beams spectral, spatial, and power characteristics, all of which limit the devices utility in the above mentioned application areas. The background of U.S. Pat. No. 5,337,328 to Lang et al. describes these limitations as arising from nonuniformities in spatial carrier densities and subsequent filamentation in broad area gain devices. In order to provide lateral mode control in DFB and DBR lasers, a lateral real refractive index waveguide, also referred to as a single mode filter, is typically introduced with a width of about 1 .mu.m to about 5 .mu.m restricting laser operation to a single lateral mode and stabilizing the device performance.
Despite the single-spatial and single-spectral mode control in DFB and DBR lasers, these devices are limited in output power to less than a couple hundred milliwatts due to carrier density limitations, due to optical power density limitations at the facets as a result of the narrow extent of the lateral waveguide, and due to significant internal losses of reflected light in the cavity propagating at large angles into nonpumped regions of the device where it is absorbed. In U.S. Pat. No. 5,103,456, Scifres et al. describe a master oscillator power amplifier (MOPA) device with a broad beam output that circumvents the aforementioned power limitations, yet provides comparable mode control and beam quality. Power in the output of this distributed reflector type laser has been enhanced, particularly for optical pumping applications, by means of forming a combination DBR single mode oscillator with a diverging or flared amplifier section forming the MOPA device. MOPA devices provide for more efficient pumping at higher power levels leading to greater signal amplification.
Present day MOPA devices, DFB lasers, and DBR lasers all exhibit certain characteristics that are unsuitable for signal and pumping applications. The geometric arrangement of the active stripe and gratings in these devices, for example, make the devices susceptible to the formation of parasitic grating-coupled modes and parasitic Fabry-Perot modes. Since the active stripe in these devices is oriented perpendicular to the end reflectors and the gratings support mode propagation along a path that is collinear with Fabry-Perot modes, these devices are susceptible to feedback noise that can change the operational characteristics of their output. In broad area lasers, such as a broad area DFB or DBR devices, or MOPA devices, all of which provide larger power outputs, multiple lateral modes may also operate and individual light filaments may develop along the path of propagation of radiation causing more stable operation at points of developed light filamentation. As a result, more spatial modes may develop resulting in nonuniform intensity in the output beam.
In order to eliminate or otherwise circumvent these problems, laser resonant cavities that include a wavelength selective mechanism in the form of a an angled distributed reflector have been proposed in the art to function, in part, as a spatial and spectral filter to improve spatial and spectral beam quality. Such devices are also referred to as angled grating laser sources or .alpha.-DFB laser sources which have a broad area, angular resonant distributed reflector along at least a portion of the length of the optical resonant cavity with the grating reflector oriented at an angle relative to cavity end reflectors which define the resonant optical cavity. Such .alpha.-DFB laser sources are disclosed in the aforementioned U.S. Pat. No. 5,337,328 to Lang et al., which is owned by the assignee herein and is incorporated by its reference.
In reference to the shortcomings described above relative to DFB, DBR, and MOPA devices, the angled distributed reflector in .alpha.-DFB lasers provides a means to achieve spatial and spectral mode control in a broad area laser resonant cavity and, in addition, simultaneously provides suppression of filamentation as well as suppression of parasitic Fabry-Perot mode competition due to utilization of a preferred geometry that is noncollinear with Fabry-Perot modes.
An example of a resonant distributed reflector that provides very narrow, i.e. selective, spectral and spatial band-pass filtering is a Bragg grating. In a "reflector" configuration, in which light is confined to a region by the gratings, unwanted spectral and spatial components of the light are transmitted through the grating. Such a configuration is well suited to the purposes of providing light output of a single spatial and spectral mode at a given wavelength of operation. Differing purposes may call for a weak grating implementation of a resonant distributed reflector or for a notch filtering configuration. However, the following discussion concentrates on the Bragg grating implementation in order to remain within conceptual framework established in conjunction with this disclosure.
A Bragg grating is a comprised of a periodic or nearly periodic array of partially reflective structures or periodic perturbation that are interposed into the light path within the coherent source. A Bragg grating can be formed by periodic variations in index of refraction, gain, loss, or a combination thereof, and can be formed in a semiconductor bulk medium of the laser structure, within a planar waveguiding structure, within an optical fiber, or within a bulk optical medium by means well known in the art. Similarly, a Bragg grating may not necessarily be strictly periodic, but may include phase shifts, interrupted regions where the grating is enhanced, diminished, or eliminated, variations in grating strength (apodization), variations in period (chirped), or multiple periods, all of which permit modification of specific aspects of the spectral filtering function performed by the grating and which are all generally known in the art.
If the Bragg grating is formed in a bulk material, the periodic or near periodic structures are planar parallel or near parallel surfaces, while if the Bragg grating is formed in a planar waveguide, the periodic or near periodic structures are parallel or near parallel linear features formed within the waveguide. Collectively, these periodic or near periodic structures will be referred to as "grooves" of the grating pattern or grating.
In the Bragg grating, the phase-matching condition must be closely maintained, i.e., the relationship EQU k.sub.out .apprxeq.k.sub.in +mk.sub.B, m=.+-.1, .+-.2, .+-.3(1)
is closely maintained or approximated, where k.sub.in is the wave vector of the light before reflection at the grating, k.sub.out is the wave vector of the light after reflection from the grating, and k.sub.B is the wavevector of the grating itself. Equation (1) is referred to and is known as the Bragg condition.
A used herein, the term, "angled distributed reflectors", is intended to include, but is not limited to, any structure composed of a collection of periodic or near periodic partial reflectors, which cause the light reflect therefrom, at least in part, due to a partial variation in the equivalent refractive index for light propagating in the region of the reflectors. Gratings and other similar perturbations are, therefore, a type of angled distributed reflector and they include pattern structures or a periodic perturbation employing etched grooves, periodic index variations, antiguides, dielectric stacks, periodic gain distributors and periodic loss distributors. Other angled distributed reflectors may be internally or externally formed optical cavity formed mirrors and the like. A Bragg type of grating is referred to throughout this description since it is a simple means of describing the multiple wave interaction that occurs between the propagating light and the grating. Moreover, the subject of this invention is not intended to be limited just to "light" waves, fixed gratings or strict Bragg gratings, as other sources of radiation or wave phenomena, other than "light", will work equally as well, and the gratings may be frequency tunable and need not precisely satisfy the Bragg condition or to be within the Bragg regime of diffraction. A Bragg grating pattern may be interposed in the light path with the grooves disposed perpendicularly to the light path, in which case the light that is reflected is directed back onto itself at a narrow range of angles and/or frequencies, such as in the case of broad area devices disclosed in U.S. Pat. Reissue No. 35,215 to Waarts et al. On the other hand, the grating grooves may be disposed at an angle to the light path, in which case, the light is deflected angularly over a narrow range of angles and frequencies. Examples of gratings employing an angled gratings are disclosed in U.S. Pat. No. 5,337,328 to Lang et al.
In angled grating or .alpha.-DFB laser sources in which a distributed Bragg reflector is employed, the reflectors are used not only to filter out undesirable light from the resonator, but also to manipulate the direction of travel of the desirable light. As shown in FIG. 1, when light 10 is incident upon an angled Bragg grating 12, which is the pumped gain region of laser 14, and the light closely satisfies the Bragg condition for reflection, then at any given position within the grating, there may be a mixture of deflected and undeflected light propagating within the grating in two different directions designated as 10A and 10B. As best seen in FIG. 2, light 13A at a designed wavelength traveling perpendicular to the resonant cavity end reflectors will be diffracted by the grating in directions and 13B. A second event of this diffraction is that the light is diffracted again in a direction normal to the cavity end reflectors and is now laterally displaced from its original path as indicated at 13A'. Note that 13A' and 13B' designate light in each of the two waves near each facet of the device, although, for simplicity, these wave vectors are only drawn near facet 18. Efficient operating of the coherent light source requires control over the amounts of light traveling in each direction 10A and 10B within grating 12, as shown in FIG. 1. For example, in the angled-grating laser 14 shown in FIG. 2, an angled grating 16 is disposed between two spatially separated, parallel reflectors 18 and 19 to form a pumped laser stripe 14A which is illustrated as a shaded area. The mean optical axis 15 of resonator 17 is basically perpendicular to facets 18 and 19 and the grooves of grating 16 are disposed at an angle .theta..sub.1 relative to this mean optical axis 15. By "mean optical axis", we are referencing an arbitrary axis in relation to resonator 17 that is positioned approximately within and oriented approximately along the two grating-coupled waves, one at 13A comprising a wave propagating parallel to the facet normal and the other at 13B comprising a wave propagating at an angle with respect to the facet normal. The utilization of mean optical axis 15, which is perpendicular to facets 18 and 19, aids in discussing the present invention relative to the existing art relating angled distributed reflector optical devices. Within resonator 17, light that strikes the facet at 18A is reflected perpendicularly from the facet back into the cavity with a portion 13D thereof also exiting from the low reflecting (LR) facet 18 as shown. Still another portion 13B diffracts off of gratings 16 approximately at a light beam angle, 2.theta..sub.1, from optical axis of resonator 17. Thus, at either end of laser 14, the light beam consists of a mixture of light propagating parallel, such as at 13A and 13A', and light propagating at angle, 2.theta..sub.1, with respect to the facet normal such as at 13B and 13B', along mean optical axis 15 of resonator 17. Upon reflection from the facets 18 and 19, a portion of the light propagating parallel to mean optical axis 15 and will return back into the resonator still parallel an optical axis 15 and thus, will approximately satisfy the Bragg condition. However, upon reflection from facets 18 and 19, the portion of the light propagating approximately at angle 2.theta..sub.1 will be reflected into a direction that no longer satisfies the Bragg condition for reflection from the grating and, as a result, is lost in unpumped regions of laser 14, as illustrated at 13C. This lost light greatly diminishes the efficiency of laser 14 and is one principal reason why these angled grating lasers have not come into practical and commercial application even though they may operate with better spatial and spectral qualities compared to conventional and presently employed DFB, DBR and MOPA laser sources.
In the ideal device, the mode propagating perpendicular to facets 18 and 19 self-selects so that the total power at the lasing wavelength is carried by the first wave 13A' of the two-grating coupled waves 13A', 13B', which wave 13A' is propagating substantially parallel to the facet normal. Ideally, no power is present in the second wave 13B' propagating at an angle with respect to the facet normal. These coupled waves are both nearly Bragg matched to the gratings and continuously exchange power along the path of the pump stripe. Due to imperfections, for example, in the grating uniformity, the optical power is not completely coupled into the parallel first wave 13A' near or at the facets 18 and 19. Thus, at these cleaved facets 18 and 19, a portion of the light is lost from the optical cavity. In the case of an .alpha.-DFB laser, for example, this lost light is significant at higher operational powers and can lead to bleaching of the unpumped region, heating and feedback into the laser pumped region at the laser pump stripe, all of which disrupt laser performance.
While the configuration of FIG. 2 describes a laser device in which light travels bi-directionally through an angled reflector, an angled reflector may also be used in a single-pass configuration, e.g., as an optical amplifier (pumped gain medium) or as a passive spatial filter or tuning element (unpumped medium), such as illustrated by optical devices 20 in FIG. 3. Collectively, therefore, all these active and passive optical devices can be broadly referred to as "optical device(s)".
In FIG. 3, optical medium 20 comprises an angular grating pump or stripe region 24 wherein, as light 22 enters grating region 24 through rear facet 23, it is deflected in two different directions 27A and 27B along mean optical axis 27 so that a part of the light exits parallel with the direction of input or, as seen at 28, while another part is divided between a portion 29A in the direction of the deflected light 27B or a related direction as dictated by the law of refraction through the facet, which is lost in space, and a portion 29B in the reflected direction, which is lost into the unpumped region of the device.
It is the object of the present invention to improve the efficiency of optical devices in the form of lasers, amplifiers, passive filters and other optical waveguide devices employing angled distributed reflectors in their optical cavities so that they are more efficient in spatially and spectrally wave component selection.
It is the further object of the present invention to provide a means of improved control over the coupling efficiency between light beams propagating within a optical device via an angled distributed reflector structure and a wave transitional boundary and preferential coupling mechanism within the optical device.